Apparatus for anodizing aluminum surfaces



Aug. 25, 1959 N. MosTovYcH ETAL 2,901,412

' APPARATUS FOR ANODIZI NG ALUMINUM SURFACES Filed Dec. '9, 1955AMPS,VOLTS AM P$ VOLTS II IIIVIII III Ill/l INVENTOR.

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H m YS W m SB v M m AD %N H WE. L A Y B ATTOBNEY United States Patent DAPPARATUS FOR ANGDIZING ALUMINUM SURFAtCES Application December 9, 1955,Serial No. 552,182

1 Claim. (Cl. 204211) The present invention relates to a process andapparatus for anodizing the surfaces of aluminum or aluminum alloybodies.

In commercial operations for anodizing aluminum surfaces for generaluse, direct current is passed through an electrolyte of the type whichhas a limited dissolving action on the oxide layer. Under the influenceof the electric current, oxidation at the aluminum metal surface underthe oxide coat is constantly occurring, while at the same time theelectrolyte is dissolving oxide principally from the exterior surface ofthe oxide layer, so that the oxide layer produced is porous.

The thickness of the oxide layer thus produced is generally proportionalto the quantity of electricity passed through the electrolyte from thecathode to the anode. Therefore, to produce a rapid or high rate ofoxide formation, a high current density should be employed.Unfortunately, a high current density burns the anode surface; hence, toprevent burning, a relatively low current density normally must be used.Somewhat higher current densities can be employed in this operation ifthe electrolyte or the aluminum surface is cooled, but this involves theexpense of cooling systems and consequent higher cost of equipment andoperation.

It has been proposed to employ an alternating anodizing current byconnecting an alternating current source across the anode and cathodebut such method is not efficient. The anodized aluminum at the anodeacts as an imperfect rectifier or asymmetrical conductor, allowing arelatively low current flow in one direction from cathode to anode and arelatively high current flow in the opposite direction. The low flowproduces an oxide coating, the high inverse flow does not. A.C. current,therefore, is more expensive and less productive than DC. current.

The principal objects of the present invention are: to provide a novelmethod of, and an apparatus for, anodiz ing aluminum which permits theuse of current densities vastly higher than heretofore found possible;to provide a method and apparatus which makes possible the efficient useof alternating current, or its equivalent, for anodizing purposes; andto provide one which is simple, effective and relatively inexpensive.

All of the objectives of our invention can be achieved by electricallyconnecting a conventional anodizing tank to an alternating currentsource in series with an imperfect rectifier or asymmetrical conductorwhich permits the desired flow of an anodizing current of high currentdensity in one direction and prevents the abnormal flow of current inthe opposite direction. The imperfect rectifier may be in the form of asecond anodizing tank serially connected in back-to-back relationshipwith the first tank so that all current flows in one direction operateas an anodizing current in the first tank and an inverse current in thesecond tank, while all current flows in the opposite direction operateas an inverse current in the first tank and an anodizing current in thesecond tanks 1 and 2 2,901,412 Patented Aug. 25, 1959 ice tank. Eachtank thus serves, during its anodizing interval, to restrict themagnitude of the current flow through the circuit and thus prevents theflow of an abnormally high current density through the other tank duringthe non-anodizing or inverse current interval of the other tank.

With two tanks connected in series, and with an alternating current ofextremely high current density flowing through both tanks, aconsiderable amount of power is wasted in each tank during each of itsinverse current alternations or intervals. This can be substantiallyreduced, in accordance with our invention, by substituting, for thesecond tank, an imperfect rectifier of they shunt resistance type whichpermits the unrestricted flow of anodizing current through the rectifierproper at little power loss, but restricts the flow of inverse currentto the shunt resistor which can be designed or adjusted to reduce thecurrent flow to a relatively small value. This flow of inverse currentis important. We have found that, to permit the use of an extremely highanodizing current density over substantial periods of time, it isnecessary to have an inverse current flow apparently to causedepolarization. We have also found, however, that the magnitude of theinverse current is not important; hence, it may be restricted to a lowvalue as above indicated.

It will be appreciated that the use of an imperfect rectifier in serieswith an alternating current source is a simple and effective way ofproducing an anodizing alternation of high current density and aninverse alternation of low current density and that this form of A.C.reduces the Waste of power to a minimum. The use of commerciallyavailable A.C. is preferred in the practice of this invention because ofits ready availability but, obviously, we may obtain the same end resultwith another type of alternating current such as results when A.C. issuperimposed upon DC. with the A.C. and DC. magnitudes being such as toproduce a large anodizing current amplitude and a relatively smallinverse current amplitude.

The method of and the apparatus for practicing the present invention isexplained hereinafter in connection with the accompanying drawingwherein:

Figure 1 schematically illustrates a pair of anodizing tanks connectedin series with each other across an A.C. source;

Figure 2 illustrates the balanced current flow obtained in thearrangement of Figure 1 and also indicates the unbalanced current flowheretofore obtained;

Figure 3 schematically illustrates a single anodizing tank connectedacross an A.C. source in series with an imperfect rectifier of the shuntresistance type;

sources of A.C.

The arrangement illustrated in Figure 1 includes a pair of anodizingtanks 1 and 2, each of which contains an electrolyte 3 composed ofdilute sulfuric acid or other suitable oxide-dissolving type ofelectrolyte. A strip 4 of aluminum to be anodized extends from a supplycoil, also designated 4, respectively over and under guide and submergedrollers 5 through 9 to a Wind-up roll 10, which preferably is powerdriven by suitable means, not shown, in order to unwind the coil andmove the strip. The are respectively provided with cathodes 11 and 12.These cathodes are connected to opposite terminals of the adjustablesecondary winding 13 of a conventional transformer 14 or to any othersuitable source of A.C. power.

In operation, we assume that, when the voltage is applied, each of thecurrent alternations designated 16, in Figure 2, will flow fromsecondary .13 .to cathode 11 of tank 1, thence through electrolyte 3therein to anodic strip 4 and along strip 4 to tank 2, thence throughelectrolyte 3 therein to cathode 12 and finally back to secondary 13. Inother Words, each alternation 16 flows as an. anodizing current in tank1 and as an inverse .current in tank 2. The latter tank does not offerany appreciable resistance to this fiow of current because it is in theinverse direction.

In tank 1, however, the voltage must build up to a value high enough toovercome the insulating or dielectric effect of the oxide barrier coaton its anode. Once this voltage is reached, the anodizing current willstart to flow and it will continue to flow until the voltage falls belowthe value required to overcome the oxide barrier coat.

The reverse of the foregoing operation is true for all currentalternations 17, since these flow in a direction opposite to that ofalternations 16; hence, function as an anodizing current in tank 2 andas an inverse current in tank 1. As a practical matter, a small leakagecurrent flows between alternations 16 and 17 hence, these alternationsare shown as connected to each other.

Before passing from Figure 2, it may be noted that alternation 17.,being an inverse current in tank 1, causes a power loss in that tankequal to the square of the current represented by alternation 17multiplied by the relatively low. resistance of tank 1 to an inversecurrent flow. Now, if tank 2 were omitted in Figure 1, an abnormalalternation 18 would flow in place of normal alternation 17, as aninverse current in tank 1, causing a substantially larger Waste ofpower. The extent of the current abnormality is graphically indicated inFigure 2 by the shaded portion of the alternation 18. Accordingly, ourinvention, as illustrated in Figure 1 in addition to its simplicity andeffectiveness, has the further advantage of reducing the power loss asabove explained.

It will, of course, be understood that each alternation 16 produces apower loss in tank 2 of Figure 1, while each alternation 17 produces acorresponding power loss in tank 1. In accordance with our invention,these power losses may be drastically reduced by substituting for, say,tank 2, an imperfect rectifier of the shunted resistor type which isillustrated in Figure 3 and which reduces the inverse power-wastingcurrent as indicated in Fig. 4.

In Figures 3 and 4, an anodizing current alternation 16 again flows fromsecondary 13 through tank 1 to and along strip 4 and thence throughrectifier 21 back to the current source. The inverse current alternation19 flows in the reverse direction but, since it cannot flow throughrectifier 21, it must flow through the shunt resistor 22 which can beadjusted to reduce the inverse current to a value which, in relation toalternation 16, is relatively small.

The modification shown in Fig. is identical to Fig. 1 except that arectifier 25 of the shunt resistor type is introduced between cathode 11of tank 1 and the corresponding terminal of the secondary 13, anothersimilar rectifier 26 is similarly introduced between cathode 12 and itsterminal of secondary 13 and the mid point 27 of secondary 13 isconnected through line 28 to strip 4. With this arrangement, ananodizing current, such as alternation 16 flowing through tank 1, willautomatically divide itself between mid point line 28 and tank 2 so thatonly a small portion of the current will flow through tank 2 as aninverse current, most of the current flowing directly through line 28 tothe secondary 13. During a reverse alternation, say 17, causing ananodizing current to flow through tank 2, the current will again divideso that most or" it returns through line 28, only a small inversecurrent passing through tank 1. The arrangement shown in Figure 5 is theequivalent of a 2- phase arrangement.

Figure 6 indicates how Figure 5 can be converted into a 3-phasearrangement. This conversion simply involves: substituting, fortransformer 14, a 3-phase transformer 14a having a secondary 13a, whichis shown as a Y-type but may be of any other equivalent type; extendingthe aluminum strip 4 through another anodizing tank 30; andconnectingits cathode 31 through an imperfect rectifier 32, of theshunted resistor type, to one lead terminal of the 3-phase secondary13a. The other lead terminals of the secondary 13a and its common point27a are connected in Fig. 6 in the same manner as the corresponding leadterminals and the common point 27 of secondary 13 are connected in Fig.5.

The operation of the embodiment shown in Figure 1 is illustrated by thefollowing specific examples:

Example 1 Temperature of bath 70 F. Electrolytesulfuric acid 15 to 20%.Transformer primary voltage, v. Current in each bath, 40 amp. Immersedarea in each bath, 40 sq. in. Voltage drop between cathode 11 and strip4:

(a) In tank 1, 14 v., and (b) In tank 2, 14 v.

The aluminum strip treated was 380 alloy, and its speed at the beginningof treatment was 4% lineal inches per minute. Due to the manner ofwinding the strip on a roll of gradually increasing diameter, the speedof the strip at the end of the treatment was 5 /2 lineal inches perminute. The connections of the cathodes to the transformer were adjustedas required to maintain the current at 40 amp.

The calculated current density in each tank was 144 amp. per square footof anode surface. The anodized film produced showed no evidence ofburning or other defects. Its average thickness was about .000 18 inch.

Example 2 In a second example, conditions employed were the same as inExample 1, except as here indicated:

Immersed area was 12 square inches. Speed of strip varied from 6% inchesper minute at beginning to 6% inches per minute at end. Current appliedvaried from about 42.5 amp. at the beginning to about 30 amp. at theend. The voltage drop in tank 1 between strip and cathode was 16 v., andin tank 2 was 22 v. The current density employed varied from about 510amp. per sq. foot of anode at the beginning to 360 amp. per sq. foot atthe end. The anodized film is opaque, greenish grey in color, and showedno evidence of burning. Its average thickness was about .00030 inch.

Having described our invention, we claim:

An anodizing apparatus comprising: a first unit including a firstshunt-resistor rectifier and a first tank for containing an electrolytefor anodizing an aluminum surface immersed in said electrolyte, saidtank having cathode and anode terminals; a second unit substantiallysimilar to said first unit, said second unit including a secondshunt-resistor rectifier and a second anodizing tank having cathode andanode terminals; an alternating current source having first and secondpower terminals and a common terminal; means electrically con nectingboth anode terminals to said common terminal; and means electricallyconnecting said first power terminal serially through the firstrectifier to the cathode terminal of the first tank and said secondpower terminal serially through the second rectifier to the cathodeterminal of the second tank, each rectifier constituting a means forrestricting the inverse current flowing through it to a valuesubstantially less than that of the anodizing current flowingtherethrough.

(References on following page) UNITED STATES PATENTS Mershon Aug. 30,1921 Coursey et a1. Nov. 9, 1937 Ruben Oct. 30, 1945 Odier Feb. 13, 1951Sherwood Apr. 10, 1956 6 FOREIGN PATENTS Great Britain June 9, 1937Great Britain May 5, 1938 Germany Oct. 31, 1939 Norway Apr. 15, 1952

